I made changes to the theme folder and now the website won't respond

I tried to create a new theme by copying the Sugar5 folder and calling it "New theme". I then changed the name to "New theme" and the desciption to "Chris's new theme" in the "themedef.php" file. Then I changed a background colour in the "style.php" file. I saved the changes and then in the Admin-> Themes page I selected my "New theme" from the dropdown menu and clicked save.

All that displays now is: "Bad data passed in; Return to home" - I cannot navigate to any part of the website to change it back as it's just a blank page with the above text!

I tried deleting my "New theme" fodler but that hasn't made any difference. Does anyone know a fix for this?